In order to avoid of carbon monoxide gas poisoning, to control the burning process, or to provide the pre-alarm of a fire disaster, semiconductor gas sensor detecting carbon monoxide gas at its low concentration by the utilization of the electric conductance changes of a metal oxide semiconductor in carbon monoxide atmosphere, are used. At present, all of the semiconductor gas sensors for detecting carbon monoxide gas are heater sensors. The TGS 203 type gas sensor manufactured by Figaro Engineering Inc. of Japan is operated in the mode of high and low temperature heating cycles; it detects the carbon monoxide gas at 80.degree. C. and the gas molecules are then desorbed at 300.degree. C. The major raw material of this kind of sensor is SnO.sub.2, with additives of 0.4% by weight of Pd and 50% by weight of X-Al.sub.2 O.sub.3. (N. Murakami, K. Takahata and T. Seiyama, Proc. 4th Int. Conf. Solid-state Sensors and Actuators, Tokyo, Japan,
CN92110578.9 (Changchun Institute of Applied Chemistry, Chinese Academy of Sciences) discloses "A process for preparing a room temperature carbon monoxide gas sensor". According to the process of CN92110578.9 SnO.sub.x is used as the base material, while Al.sub.2 O.sub.3, MgO, Pt, Pd and ThO.sub.2 are used as additives for preparing the said gas sensor, wherein, the value of x is in the range of 1.0-1.6. The carbon monoxide gas sensor prepared according to this process can detect carbon monoxide at room temperature; the surface temperature of the sensor is low, the energy consumption of the sensor is low; the application circuit is simple. However, this gas sensor has the following disadvantages: long oscillation cycle period (50-100 seconds), and short service life. In addition, since ThO.sub.2 is used, the sensor obtained by the said process has some radio-activity.
CN89102921.4 (Changchun Institute of Applied Chemistry, Chinese Academy of Sciences) discloses "A room temperature semiconductor gas sensor". The said semiconductor gas sensor is mainly used for detecting coal gas and liquefied petroleum gas. The ratio of the raw materials used in this sensor is as follows: SnCl.sub.4 /SnO.sub.2 is in the range of 0.5-1.5 (weight); oxides of rare earth metals SnO.sub.2 is in the range of 0.5-1(weight), wherein, the said oxides of rare earth metals are only LaO.sub.2 and CeO.sub.2. Similarly, the said gas sensor has the disadvantages of long oscillation cycle period (50-100 seconds) and short service life(less than 45 days).